Numerous surveying devices are known for surveying one or more target points. Generally known modern examples of such surveying devices are laser trackers, that is to say measuring apparatuses which are designed to continuously track a target point and to determine the position of this point in coordinates, or surveying devices specifically tailored to geodetic surveys such as tachymeters and total stations. The latter are also referred to as an electronic tachymeter or computer tachymeter and have a number of different subsystems for performing the surveying tasks in a highly automatic or automated manner. A geodetic surveying device from the prior art is described in EP 1686350, for example. In this case, the distance and direction or horizontal and vertical angle of a surveying device, the absolute position of which is known, with respect to the target point to be surveyed are recorded as spatial standard data.
In many applications, points are surveyed by placing specially configured target objects there or mounting them on a movable vehicle. These consist of, for example, a plumb rod with a retroreflector (for example a 360° prism) for defining the measurement section or the measurement point. However, surveying systems which operate without a reflector are also possible, as are described in the European patent application with the application number EP 10168771.3, for example.
The accuracies required during surveying in road or tunnel construction, for example, are in the millimeter range, even at great distances between the surveying device and the target point of more than 100 meters, with accordingly high requirements imposed on the surveying device and possibly on the retroreflective target object which is used to mark a target point to be surveyed.
In order to sight or target the target point to be surveyed, surveying devices of the generic type have a telescopic sight, for example an optical telescope, as the sighting device. The telescopic sight is generally rotatable about a vertical standing axis and about a horizontal tilting axis relative to a base of the measuring device, with the result that the telescope can be aligned with the point to be surveyed by pivoting and tilting. Modern total stations also have means for motorizing the target optics and for automatic target sighting/fine targeting and target tracking, abbreviated using ATR (Automatic Target Recognition) below. A description of such an ATR system is found, for example, in the paper by Kirschner, H. and Stempfhuber, W.: The Kinematic Potential of Modern Tracking Total Stations—A State of the Art Report on the Leica TPS1200+. 1st International Conference on Machine Control & Guidance 2008 (retrieved on Mar. 4, 2015 from www.mcg.ethz.ch/papers/Kirschner_Stempfhuber05.pdf).
The ATR systems according to the prior art have means for emitting an illumination beam and for capturing at least one part of the portion of the illumination beam reflected by a target, for example a point in the environment or a reflective prism. The illumination is usually carried out in this case by continuously emitting short illumination beam pulses or illumination beam flashes, wherein the illumination beam is, for example, a divergent laser beam and laser pulses are accordingly continuously emitted as illumination beam flashes. In this case, the reflected laser light is imaged onto an image sensor, for example a CMOS 2D chip, as a reflected spot (light spot). Depending on the deviation of the orientation of the optical targeting axis from the direction to the target object, the impingement position of the reflected radiation on the ATR sensor also deviates in this case from a central sensor area position, that is to say the light spot of the ATR illumination beam retroreflected at the prism on the ATR area sensor is not in the center of the ATR area sensor and therefore does not impinge on a desired position defined, for example using calibration, as that position which corresponds to the optical target axis. During a fine targeting function, the position of the target relative to the optical targeting axis is therefore inferred using the position of the reflected spot or the exposed pixels on the image sensor. In the case of a deviation, the fine targeting function is usually used to slightly adjust the orientation of the sighting device in a motorized manner such that the ATR measuring beam retroreflected at the prism impinges on the ATR area sensor in a highly precise manner in the center of the sensor area, that is to say the horizontal and vertical angles of the sighting device are iteratively changed and adjusted until the center of the reflected spot coincides with the desired position on the ATR area sensor. In order to ensure the functioning of the automatic fine targeting, it is necessary, before the function starts, to align the sighting device at least approximately with the target reflector in such a manner that the ATR illumination beam also impinges on the prism and, having been reflected from there, on the ATR area sensor. For this purpose, manual targeting of the target reflector on the basis of visual judgment can be carried out in advance, for example, or an automatic coarse targeting function can be carried out.
In addition to the ATR fine targeting functionality, an automatic target tracking functionality can also be provided in a similar manner and using the same ATR components, as is the case, in particular, in laser trackers, but also in modern total stations.
During target tracking, the position of the moving target is determined continuously or at very short intervals of time. The measurement/the surveying device follows the movement of the geodetic target. After ATR fine targeting has been carried out, the sighting device therefore continues to be tracked to movements of the target “live” and accordingly quickly such that the center of the ATR reflected spot still remains as accurately as possible and always at the desired position on the ATR area sensor. Reference is then often made to “locking on” to the target or to the fact that the target is “locked on”.
Problems may arise in this case if the target moves so suddenly and quickly that it disappears from the field of view of the ATR detector (that is to say ATR measurement radiation reflected at the target no longer impinges on the ATR area sensor). Other causes which make it difficult or impossible to recognize the target or to fine target and track the target and restrict the maximum operational range are environmental influences. Such disruptive environmental influences are, in particular, climatic influences which influence the optical path such as rain, fog or heat shimmer. Extraneous reflections are also disruptive, that is to say, for example, light which, in addition to the measurement radiation reflected by the target, is imaged onto the image sensor. Such extraneous reflections are caused by extraneous light or beam sources such as direct solar radiation or indirect solar radiation, that is to say solar radiation reflected by road signs or glass surfaces, or headlights of construction vehicles. Disruptions are problematic, in particular during target tracking, since they often result in the locking on to the target reflection being lost, which makes it necessary to carry out time-consuming locking-on again.
In order to avoid disruptions caused by climatic influences, total stations according to the prior art provide the option of setting parameters of the ATR system or of the fine targeting and tracking function in a weather-dependent manner by means of manual configuration by the user. In this case, however, the total station is only roughly adjusted to the present climatic conditions, with the result that there is usually no optimum choice of parameters, and this is also associated with additional effort for the user.
In order to eliminate extraneous reflections and to distinguish between the reflection of the target and extraneous reflections, that is to say in order to distinguish between the reflected spot and other light spots on the sensor which stem either from external light sources or from an illumination beam component which is not reflected at a target, the prior art of automatic target recognition discloses recording two images using the image sensor with a constant alignment and position of the surveying device and the target, wherein the illumination beam is not emitted when recording one of the two images. As a result, a reflected spot can be detected only in one of the images, with the result that the reflected spot can be distinguished from noise or extraneous reflections and the target can be recognized by means of image processing with the formation of differences between the two images. However, this method is possible only for the static situation, that is to say without a relative movement of the target or extraneous reflection source with respect to the surveying device. Such a method is disclosed, for example, in the White Paper “Direct Aiming Station” by Topcon, retrieved on Mar. 4, 2015 from www.topcontotalcare.com/files/2013/7525/6386/DS_WP_P-180-2_TE.pdf.
WO 1997/10517 A1 discloses a target recognition method with modulation of the polarization of the emitted light. The disadvantage here is the additional outlay for the polarization, inter alia in the form of polarization means which additionally must be at least partially arranged on the target reflector, with the result that conventional retroreflectors cannot be used. WO 2011/098131 uses an apparatus having two differently arranged radiation sources and at least two image sensors, wherein reflections are distinguished using the signals from the image sensors during illumination with reflected light from the first radiation source and the signals during illumination with reflected light from the second radiation source. The disadvantage here again is also the additional outlay on means, this time in the form of the additional light source and image sensors. In addition, disruptive climatic influences are not dealt with in the two documents mentioned.